Strip transmission line crossover having reduced impedance discontinuity



Sept. 17, 1963 BUTLER 3,104,363

STRIP TRANSMISSION LINE CROSSOVER HAVING REDUCED IMPEDANCE DISCONTINUITY Filed July 25, 1960 L\\\\\\\l\\\\\\\\\\\V Jesse L. Butler 8 3 i 3 INVENTOR jnwwywaam ATTORNEY United States Patent 3,104,363 STRIP TRANSMISSION LINE CROSSOVER HAVING REDUCED IMPEDANCE DISCONTHNUITY Jesse L. Butler, Nashua, N.H., assignor to Sanders Associates, Inc., Nashua, N.H., a corporation of Delaware Filed July 25, 1950, Ser. No. 44,937 2 Claims. (Cl. 33384) This invention relates to a transmission line crossover for use in strip transmission lines. More particularly, it relates to a crossover in which the center conductors of a pair of intersecting lines are reduced in width, thereby minimizing coupling between the lines while at the same time maintaining their characteristic impedances substantially uniform.

Most electric circuits having a fair degree of complexity require signal paths to cross :over other signal paths without appreciable transfer of energy from one path to the other. At low frequencies, this problem can easily be solved by use of a three dimensional construction in which one line arches over another at each crossover. At higher frequencies, where transmission lines are used and, more specifically, in the case of strip lines in which printed or etched circuits are used, a three dimensional crossover construction in which one transmission line arches over the other is relatively expensive to fabricate. Furthermore, it hass a space requirement which works against the present-day trend to miniaturization.

In the case of strip lines, another possibility is to arch the center conductor of one of the intersecting lines over the center conductor of the other line within the confines of a pair of ground plane conductors common to both lines. Fabrication of a crossover in this manner is relatively inexpensive, and the space requirement is little greater than that of a complex of non-crossing strip lines, using common ground plane conductors. However, there is significant capacitive coupling between the lines, re sulting in a serious cross talk problem, and, furthermore, the characteristic impedance of each line is affected by the proximity of the other line at the crossover, with a consequent deleterious effect on the transmission properties of the lines.

Accordingly, it is a principal object of my invention to provide an improved crossover adapted for the transposition of two strip transmission line signal paths.

A further object is to provide a crossover of the above type substantially con-forming to a two dimensional layout and therefore adapted for construction by printed circuit and like techniques.

A still further object of my invention is to provide a crossover of the above character having relatively little cross talk between the intersecting signal paths.

Yet another object of my invention is to provide a crossover of the above character substantially free of discontinuities in the characteristic impedances of the crossing transmission lines.

A still further object of the invention is to provide a crossover of the above character adapted for relatively inexpensive fabrication.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

In general, a crossover incorporating the principles of my invention comprises a pair of inner conductors disposed substantially midway between a pair of outer or ground plane conductors in a strip transmission line configuration. The inner conductors are offset slightly from their central positions between the ground plane conductors so that they may cross over each other without physical contact between them. In the region where the conductors pass over each other, they are reduced in width, and this largely mitigates most of the problems otherwise encountered in a crossover of this type. More specifically, as shown below, it reduces coupling between the two transmission lines in the crossover and at the same time eliminates discontinuities in their characteristics impedances, thereby eliminating undesirable reflections and at the same time minimizing transfer of energy between the lines.

For a fuller understanding of the nature and objects of the invention, reference should be had to the -following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a transverse section of strip transmission line, showing the configurations of the electromagnetic fields between the conductors,

FIGURE 2 is a perspective view of the inner conductors of a pair of transmission lines in a crossover embodying the principles of my invention, and

FIGURE 3 is a sectional view of a crossover of the type shown in FIGURE 2.

In FIGURE 1 there is illustrated the field distribution in a typical strip transmission line. The line has an inner conductor 10 situated between and parallel to a pair of outer ground 'plane conductors 12 and I4. The conductors 10, 12, and 14 are fiat and may be quite thin. For example, they may be formed of foil bonded to dielectric material (not shown) filling the space between them. At an instant of time when the conductor 10 is positive with respect to the ground planes 12 and 14 and the current in the conductor 10 is in the direction of the arrow 15, the field distribution in the transmission line is as shown in FIGURE 1, with the solid arrows representing the electric field E and the dash lines representing the mag netic field H.

The field configuration of FIGURE 1 is indicative of the TEM propagation mode. However, it is possible to transmit other modes on the line under certain conditions. For example, if the inner conductor 10 is offset from its nominal position midway between the ground planes 12 and 14, the ground planes will be at somewhat different potentials. This difference in voltage will support a parallel plate mode. Accordingly, the ground planes are shorted together by a plurality of pins '16 spaced along both edges of the inner conductor. The pins impose an equipotential conditions on the planes and thereby suppress this mode. For effective suppression, the spacing of the pins in the lengthwise direction of the line should be less than a half wavelength. Ordinarily, this spacing is on the order of one eighth wavelength or less.

Another limitation on pin spacing results from the desirability of avoiding a resonant condition in any loop defined by the ground planes and a pair of adjacent pins. A resonant loop will distort the transmission characteristics of the line as well as facilitate radiation of energy therefrom. Resonance occurs when the length of the loop is an integral number of wavelengths, and, accordingly, the distance between adjacent pins should be considerably less than the spacing providing a wavelength loop.

If either of the transverse dimensions, i.e., ground plane to ground plane or pin to pin spacing is greater than a half wavelength, a transverse electrical waveguide mode may be excited. Therefore, both these dimensions should be less than a half wavelength. There is also a restriction on the length of the circumferential path around the inner conductor 10 and passing midway between the inner conductor and the ground planes 12 and 14 and pins 16. This path should be less than a wavelength.

success the immediate vicinity of the crossover, the inner conductors are not coplanar, but rather are slightly displaced from each other; thus, an insulator fragmentarily shown at 26 may be disposed betweenv them. In the region Where they lie one above the other, the inner conductors have reduced portions 28 and 30.

Reduction in the width of the conductors 22 and 24 in the portions 28 and 30 provides several important advantages. there will be a material transfer :of energy from one trans-mission line to the other by means of capacitive coupling between the conductors 22 and 24. A number of undesirable effects stem from the energy transfer. In the first place, there is a loss of energy from the line on which the signal involved is to be propagated, and, as a corollary, there is the introduction of interference on the line to which the signal is coupled. Moreover, the loss of energy from a line is tantamount to the connection of a load across it. In the case of coupling in the above manner, this load has two parallel parts, which may be represented by (1) a coupling capacitance in series with one half the characteristic impedance of the other line (assuming matched terminations at both ends thereof); and (2) an additional capacitance between the inner conductor of. the first line and the ground plane conductors resulting from the presence of the other inner conductor in the held of the first line. Because of this loading efiect, there is a discontinuity in the characteristic impedance of each transmission line in the crossover, with undesirable reflections at this discontinuity.

The reduction in Width in the portions 28 and 3t} significantly reduces the capacitance between the two lines and thus diminishes the coupling between them. Therefore, the amount of energy lost from one line to the other is greatly reduced, and the efiect of each line on the capacitance of the other is also materially diminished. Furthermore, a decrease in width of an inner. conductor increases the characteristic impedance of the transmission line including the conductor. Thus, in my crossover, the characteristic impedance of each line is increased by the reduction in width at the same point Where it is decreased by the loading efiect noted above. Preferably, the widths are reduced by an amount which balances the increase in impedance against the decrease, so that there is no net change in characteristic impedance of either transmission line where it extends through the crossover. actual amount of the reduction in width depends on such factors as the spacing between the lines, the material of the insulator 26 and the characteristic impedance of the Assuming that there is no reduction in width,

The-

transmission line which can easily be determined experimentally by standard impedance measurement techniques.

As seen in FIGURE 3, the conductors 22 and 24 are bonded to a pair of insulators 32 and 34, on the oppo site surfaces of which are also bonded a pair of outer ground plane conductors 36 and 38. The relative dimensions have been exaggerated in this view for purposes of clarity, it being understood that all the conductors may be in the form of thin films and the thickness of the inisulator 26 a few thousandths of an inch. The widths of a the conductors 22 and 24 and thicknesses of the insulators 62 and 34 largely determine the characteristic impedances of the transmission lines.

It will thus be seen that, the objectsset forth above} among those ma'de'appare nt from the preceding description, are efficiently attained and, since certain changes may be made in the above construction Without departing from the scope of the invention, it is intended that and second strip transmission lines having first and second,

inner conductors jointly disposed between and insulated from a pair of ground plane conductors, said inner conductors being disposed at an angle to each other and substantially parallel to said ground plane conductors, said first inner conductor passing over said second inner-con doctor, said inner conductors being spaced apart and insulated from each other in the region where they lie one over the other, at least one of said inner conductorshaving a reduced width in said region, the amount of said reduction in width being such as to increase the characteristic impedance of the transmission line including said one inner conductor by substantially the same amount as it is decreased by coupling to the other of said inner conductors, each of said inner conductors extending in both directions from said region.

2. The combination defined in claim 1 in which each Handbook of Tri-Plate Microwave Components, Sanders Associates, 1956, pages 34, 35,92 and 93. 

1. A STRIP TRANSMISSION LINE CROSSOVER COMPRISING FIRST AND SECOND STRIP TRANSMISSION LINES HAVING FIRST AND SECOND INNER CONDUCTORS JOINTLY DISPOSED BETWEEN AND INSULATED FROM A PAIR OF GROUND PLANE CONDUCTORS, SAID INNER CONDUCTORS BEING DISPOSED AT AN ANGLE TO EACH OTHER AND SUBSTANTIALLY PARALLEL TO SAID GROUND PLANE CONDUCTORS, SAID FIRST INNER CONDUCTOR PASSING OVER SAID SECOND INNER CONDUCTOR, SAID INNER CONDUCTORS BEING SPACED APART AND INSULATED FROM EACH OTHER IN THE REGION WHERE THEY LIE ONE OVER THE OTHER, AT LEAST ONE OF SAID INNER CONDUCTORS HAVING A REDUCED WIDTH IN SAID REGION, THE AMOUNT OF SAID REDUCTION IN WIDTH BEING SUCH AS TO INCREASE THE CHARACTERISTIC IMPEDANCE OF THE TRANSMISSION LINE INCLUDING SAID ONE INNER CONDUCTOR BY SUBSTANTIALLY THE SAME AMOUNT AS IT IS DECREASED BY COUPLING TO THE OTHER OF SAID INNER CONDUCTORS, EACH OF SAID INNER CONDUCTORS EXTENDING IN BOTH DIRECTIONS FROM SAID REGION. 